interfacial composition, structural and thermodynamic parameters of water/(surfactant+ n butanol)/ n...
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ORIGINAL ARTICLE
Interfacial Composition, Structural and ThermodynamicParameters of Water/(Surfactant+n-Butanol)/n-HeptaneWater-in-Oil Microemulsion Formation in Relationto the Surfactant Chain Length
Mrinmoy De • Subhash C. Bhattacharya •
Satya P. Moulik • Amiya K. Panda
Received: 9 July 2009 / Accepted: 1 February 2010 / Published online: 10 March 2010
� AOCS 2010
Abstract Interfacial behavior, structural and thermody-
namic parameters of a water/(surfactant?n-butanol)/
n-heptane water-in-oil (w/o) microemulsion have been
investigated using the dilution technique at different tem-
peratures, and [water]/[surfactant] mole ratios. The cationic
surfactants used were alkyltrimethyl ammonium bromides
(CnTAB, n = 10, 14 and 16) while the nonionic surfac-
tants were polyoxyethylene (20) sorbitan monoalkanoates
(polysorbate), viz., palmitate (PS 40), stearate (PS 60) and
oleate (PS 80). The distribution of cosurfactant between the
oil–water interface and the bulk oil at the threshold level of
stability, and the thermodynamics of transfer of the
cosurfactant from the bulk oil to the interface were eval-
uated. Structural parameters such as the dimensions, pop-
ulation density and effective water pool radius of the
dispersed water droplets in the oil phase and the interfacial
population of the surfactant and cosurfactant have been
evaluated in terms of the surfactant chain length.
Keywords Water-in-oil microemulsion � Surfactant �Cosurfactant � Structural parameter �Thermodynamic parameters � Dilution technique
Introduction
Microemulsions are isotropic, thermodynamically stable
dispersions of oil in water (o/w) or, vice versa (w/o), stabilized
by a surfactant monolayer at the water/oil interface [1–9].
Also there are some bicontinuous microemulsions which are
able to exhibit exceptional properties like the highest solubi-
lization and lowest tension. The present study is dedicated to
the water-in-oil microemulsion. Usually, single-chained
surfactants require a cosurfactant for the stabilization of such
dispersions [4]. Short chain alkanols and amines are quite
often used as cosurfactants. A cosurfactant reduces the
bonding stress during the formation of a dispersed phase.
Studies on microemulsions have been the focus of attention
over a considerable period of time due to their multifaceted
application potentials [10–12]. With the advent of newer
techniques, the scope for varied physicochemical studies are
on the increase. The potential techniques include Small
Angle X-ray Scattering (SAXS), Small Angle Neutron
Scattering (SANS), Quasi Elastic Light Scattering (QELS),
also know as Dynamic Light Scattering (DLS), Nuclear
Magnetic Resonance (NMR), Time Resolved Fluorescence
Quenching (TRFQ), diffusion, electrical conductance,
solvation dynamics, etc. [12–17]. In addition to all the
cited techniques, a relatively simple and unsophisticated
method that hardly requires any instrumentation can be
used to gather very fundamental information on the forma-
tion of w/o microemulsions. This is the method of dilution
[13, 14, 17–19], which is advantageous because of its sim-
plicity. In a w/o microemulsion containing a cosurfactant,
the water phase is linked with the oil by a surfactant mono-
layer at the interface [20]. On the other hand, the cosurfactant
essentially partitions between the oil phase and the interfa-
cial region; it may even moderately or weakly partition
between the water phase and the interface as well. A stable
M. De � A. K. Panda (&)
Department of Chemistry, University of North Bengal,
Darjeeling 734 013, West Bengal, India
e-mail: [email protected]
M. De � A. K. Panda
Department of Chemistry, Behala College,
Kolkata 700 060, West Bengal, India
M. De � S. C. Bhattacharya � S. P. Moulik
Centre for Surface Science, Department of Chemistry,
Jadavpur University, Kolkata 700032, West Bengal, India
123
J Surfact Deterg (2010) 13:475–484
DOI 10.1007/s11743-010-1186-7
clear four component (water, surfactant, cosurfactant and
oil) microemulsion, can be destabilized by extra addition of
oil to make it inhomogeneous and turbid. To satisfy partition
requirement there is a depletion of cosurfactant at the oil
water interface. Adequate addition of the cosurfactant helps
to restore the system to a renewed state of thermodynamic
equilibrium. This procedure of stabilization and destabili-
zation can be performed a number of times while noting the
amounts of oil and cosurfactants required to achieve the
above described process. By this simple procedure, the dis-
tribution of the cosurfactant between the oil and interfacial
regions can be ascertained to evaluate the thermodynamics
of the microemulsion formation process [1, 21, 22]. Such an
evaluation by other chemical and physical methods is not so
easy. For details of the process and its efficacy we refer to the
works published in the literature [20, 23–25]. The thermo-
dynamics of transfer of the cosurfactant from the oil to the
interface can be quantitatively understood with the help of
mathematical relations. In addition, the structural informa-
tion on the nanodispersions of water in the oil continuum can
also be gathered [1, 21, 22].
In most of the reported studies, different kinds of oils,
surfactants, and cosurfactants have been used. Systematic
investigations with cosurfactant variation have seldom
been done. Recently, Digout et al. [26] have attempted to
understand the effect of variations of oil and cosurfactant
chain lengths on w/o microemulsion formation. In our
previous work, the effect of surfactant head-group variation
has been studied considering cationic, anionic and nonionic
surfactants having an identical alkyl chain length [27]. To
the best of our knowledge, detailed study on microemul-
sion formation by the method of dilution using surfactants
of the same homologous series is unexplored.
In the present study, the dilution method has been
employed to derive different physicochemical parameters
for the formation of water/(surfactant?n-butanol)/n-hep-
tane w/o microemulsions wherein both cationic and non-
ionic surfactants of two series have been used. The
cationics were decyl (C10), tetradecyl (C14) and hexadecyl
(C16) trimethyl ammonium bromides, and the nonionics
polyoxyethylene (20) sorbitan monopalmitate, monostea-
rate and monooleate. The dilution results have been ana-
lyzed and processed to understand the alkyl chain length
effects on the phenomenon of microemulsion formation in
the studied micro heterogeneous systems.
Experimental
Materials
The surfactants decyltrimethyl ammonium bromide (C10TAB),
tetradecyltrimethyl ammonium bromide (C14TAB),
hexadecyltrimethyl ammonium bromide (C16TAB), and three
polysorbates (PS) or sorbitan monoalkanoates esters with 20
ethylene oxide groups per molecule, i.e., monopalmitate,
monostearate and monooleate (respectively, abbreviated
PS40, PS60 and PS80 in what follows), were products from
Sigma Chemicals, USA. They were fairly pure (cationics were
[99% pure) and were further purified using standard pro-
cedures [28–30]. The surfactants did not produce a minimum
in their surface tension (c) versus concentration plots [31–33].
HPLC grade n-heptane and n-butanol were purchased from
Merck, India. Doubly distilled water with a specific resistivity
of 18 MX cm was used during the experiments.
Methods
At a given temperature, water and surfactant at a fixed
[water]/[surfactant] ratio x, were placed in a dry test tube
with a known quantity of n-heptane and then placed in a
thermostated water bath (of accuracy ±0.1 K). The viscous
and turbid solution was then titrated with n-butanol under
stirring to make it just clear. The system was given suffi-
cient time to attain equilibrium; once turbidity had disap-
peared, it was not restored. The volume of n-butanol
required to make the stable microemulsion was noted. Then
a known amount of n-heptane was added to destabilize the
microemulsion. The turbid solution thus formed was then
again titrated with n-butanol until it appeared just clear as
stated above. The volume of n-butanol added was noted
again. This stabilization–destabilization procedure was
repeated several times noting the volumes of n-butanol and
n-heptane at each occasion [20, 34, 35]. The entire pro-
cedure was carried out at different x (5, 10, 15, 20, 25) and
at different temperatures (303, 308, 313, 318 and 323 K).
Each set of experiments was repeated twice and the aver-
age values were used for data processing and analysis.
Theoretical Backgrounds
Evaluation of Thermodynamic Parameters
For a stable microemulsion, the total number of moles of
alkanol (nat ) is [20, 26];
nta ¼ nw
a þ nia þ no
a ð1Þ
where naw, na
i and nao represent the number of moles of
alkanol in water, at the interface and in oil, respectively.
Other conditions remaining the same, the ratios (1)
between the number of moles of cosurfactant (alkanol) in
oil (nao) and the number of moles of oil (no), and (2)
between the mole fraction of alkanol at the interface
(Xai ) and that in the oil (Xa
o) are fixed. For a stable
equilibrium,
476 J Surfact Deterg (2010) 13:475–484
123
k ¼ noa
no
ð2Þ
and Kd ¼Xi
a
Xoa
ð3Þ
With the help of Eq. 2, Eq. 1 can be rewritten as
nta ¼ nw
a þ nia þ kno
or;nt
a
ns
¼ nwa þ ni
a
ns
þ kno
ns
ð4Þ
where ns is the number of moles of surfactant present in the
system.
From Eq. 4, ifnt
a
nsis plotted against no
nsthen k and
nwa þni
a
nscan
be obtained from the intercept (I) and the slope (S) of the
straight line, respectively. naw is generally obtained from the
solubility of the cosurfactant (alkanol) in water [34]. The
aqueous solubility of higher alkanols than n-butanol is very
small and hence for those alkanols, naw = 0 [34, 36].
Equation 3 can also be written in terms of I and S as:
Kd ¼Xi
a
Xoa
¼ nia=ðni
a þ nsÞno
a=ðnoa þ noÞ
¼ni
að1þno
a
noÞ
noa
noðni
a þ nsÞð5Þ
or; Kd ¼að1þ SÞ
S½1þ ðI � nwa
nsÞ�¼ að1þ SÞ
Sð1þ aÞ ð6Þ
where, a ¼ ðI � nwa
ns
Þ ¼ nia
ns
ð7Þ
Kd can easily be evaluated if I, S and a are known. For
higher alkanols than n-butanol naw = 0 and a = I, then [27]
Kd ¼Ið1þ SÞSð1þ IÞ ð8Þ
The standard Gibbs free energy of transfer (DGto) of
alkanol from the oil phase to the interfacial phase is as
follows:
DGot ¼ �RT ln Kd ð9Þ
where, ideal behavior is assumed and the concentration is
expressed on the mole fraction scale.
Standard enthalpy changes of transfer (DHto) and entropy
changes of transfer (DSto) are given by the relations,
oðDGot =TÞ
oð1=TÞ
� �p
¼ DHot ð10Þ
and DSot ¼
DHot � DGo
t
Tð11Þ
However, for a nonlinear dependence of DGto on T, a
two degree polynomial equation (shown below) can be
used to get DHto [27]:
DGot ¼ aþ bT þ cT2 ð12Þ
where a, b and c are the polynomial coefficients.
The differential form of Eq. 12 helps us to evaluate
DHto. Thus,
oðDGot =TÞ
oð1=TÞ
� �p
¼ a� cT2 ¼ DHot ð13Þ
Evaluation of Structural Parameters
The structural parameters of the nanodispersion of water in
a w/o microemulsion can be determined using a simplified
structural model assuming monodispersion of the droplets
[18, 26, 37, 38].
The total volume of the dispersed microemulsion
droplets (Vd) is related in the following way:
Vd ¼4
3pR3
eNd ð14Þ
with Nd (total number of droplets) and Re (their effective
radius). Vd also follows the relation:
Vd ¼ VH2O þ VS þ V ia ð15Þ
where VH2O, Vs and Vai are the volumes of water, surfactant
and cosurfactant at the interface, respectively. These vol-
umes are related to their masses with their respective
densities [20, 39].
The total droplet surface area (Ad) is expressed as:
Ad ¼ 4pR2eNd ¼ ðnsAs þ ni
aAaÞNA ð16Þ
where AS and Aa are the polar head group areas of the
surfactant and the alkanol, respectively, and NA is the
Avogadro constant.
The values of Re and Nd are calculated from the fol-
lowing equations:
Re ¼3Vd
Ad
ð17Þ
and Nd ¼3Vd
4pR3e
ð18Þ
In a reversed micelle, the average aggregation number
of the surfactant (Ns) and the cosurfactant (Na) are obtained
from
Ns ¼nsNA
Nd
ð19Þ
Na ¼ni
aNA
Nd
ð20Þ
The effective radius of the water droplet (Rw) of
the dispersed phase including the contributions of the
amphiphile and the cosurfactant head groups is the
following,
J Surfact Deterg (2010) 13:475–484 477
123
Rw ¼VH2O þ Vh
s þ Vha
Vd
� �1=3
Re ð21Þ
where Vsh and Va
h are the volumes of the head groups of the
surfactant and the alkanol, respectively. They obey the
following relations [40]:
Vhs ¼
4
3p1=2A3=2
s Ns ð22Þ
Vha ¼
4
3p1=2A3=2
a Na ð23Þ
Results and Discussion
Thermodynamics of Dilution
Figure 1 depicts the representative linear plots ofnt
a
nsversus
no
nsfor the dilution experiments with C14TAB (Fig. 1a), and
PS40 (Fig. 1b) in terms of Eq. 4. For C14TAB the slopes
and the intercepts increase with x, whereas for PS40 the
slopes remain nonvariant but the intercepts progressively
increase with x. With increasing water pool size, more
cosurfactant is required to form a stable microemulsion
[26, 27]. Higher pool size also requires more naw for the
reason of solubility. The slopes representno
a
noi.e., the dis-
tribution of alkanol between the oil/water interface and oil
which is a constant (c.f. Eq. 2). The observed results sug-
gest increased solubility requirement of the alkanol in the
oil as x increases when C14TAB is the surfactant. For PS40
the solubility requirement remains unchanged. Nonionics
have relatively bulkier head groups (polyoxyethylene sor-
bitan moieties). With the increase in water pool size the
head groups can ‘‘open up’’ or uncoil or unfold which
requires a higher number of n-alkanols at the interface, as
the unfolded hydrophilic head group cannot provide better
coverage at the oil–water interface. On the other hand,
cationic surfactants (CnTAB) having a specific head group
area could not undergo such processes as is the case in
polysorbates. Hence Kd values for cationic surfactants do
not change significantly with the increase in x. In one of
our previous studies, we explored the dilution behavior on
different surfactants having a similar chain length with
different head groups [27]. Results are reflected through the
Kd values as represented in Table 1.
It can be seen (Fig. 2a, b) that, at all temperatures, Kd
declines faster with x for C14TAB than for PS40. But at all x,
the Kd increases faster with temperature for PS40 than for
C14TAB, where the dependence is very mild. These phe-
nomena are the same for all the cationic and nonionic repre-
sentatives. For a fixed x, Kd, on the whole, increases with
temperature (Table 1) except for the system containing
C14TAB, where Kd mildly declines with temperature. This out
of trend phenomenon needs corroboration by further studies.
For both categories of surfactants, the spontaneity of the
transfer increases with the surfactant chain length. This is
related to HLB, which declines with increasing chain
length [41]. The hydration/dehydration of the nonionic
surfactant head groups is fairly susceptible to temperature
in comparison with the cationics, which causes a difference
between the two. The observed DGto (kJ mol-1) values
vary in the range of—(2.5–7.6). Similar variations were
also reported in the ranges of—(3.1–4.5) by Bansal et al.
[18],—(5.5–9.9) by Gerbacia and Rosano [21],—(4.5–5.1)
by Kumar et al. [22],—(5.0–6.0) by Birdi [23], and—(4.0–
8.0) by Singh et al. [38]. A non-linear (two degree poly-
nomial) variation of DGto with surfactant chain length was
followed with both the cationic and the nonionic surfac-
tants. The presence of unsaturation in PS80 has little effect
on this trend. For both classes of surfactants, the enthalpy
values are found to be positive; the processes are endo-
thermic in nature (excepting for C10TAB at 303 K) while
the changes in entropy are largely positive. The processes
are fairly controlled by entropy, particularly for the non-
ionic surfactants-derived formulations. Digout et al. [26]
have reported that the presence of the surfactant at the oil–
water interface can make an entropic contribution to reduce
the energy of transfer of alkanols between the bulk oil and
the interfacial region. For cationic surfactants, the extent of
endothermicity decreases with x. However, temperature
does not produce a trend in their variation for the cationic
surfactants. For nonionic surfactants, DGto weakly varies
with temperature whereas both DHto and DSt
o increase
constantly with temperature.
Figure 3 presents the dependencies of DHto and DSt
o on
surfactant chain length (i.e., the number of carbon atoms or
Cn) at various temperatures at a constant x = 15. Both
Fig. 1 Plot of nat /ns versus no/ns according to Eq. 4 for water/
(surfactant?n-butanol)/heptane w/o microemulsion systems at 313 K
and at different x. Surfactants: a C14TAB; b PS40. x: open circle, 5;
open triangles, 10; open squares, 15; inverted triangles, 20 and opendiamonds, 25
478 J Surfact Deterg (2010) 13:475–484
123
DHto and DSt
o curves cross at C14; the sequences for the
cationics just become reversed after C14, which is the
crossing point at all x values (the results other than
x = 15 not shown). It would be interesting if results with
homologues like C10 and C18 were available. This remains
to be examined in a future study.
Table 1 Thermodynamic parameters for the transfer of n-butanol from the oil phase to the interface in water/(surfactant?n-butanol)/heptane w/
o microemulsions at different temperatures
Surfactant Temp./K Kd -DGtO/kJ mol-1 DHt
O/kJ mol-1 DStO/Jmol-1K-1
C10TAB 303 4.86 3.98 -0.44 11.68
308 4.89 4.06 0.74 15.58
313 4.87 4.12 1.95 19.40
318 5.03 4.27 3.18 23.41
323 5.09 4.37 4.43 27.22
C14TAB 303 9.47 5.66 2.82 9.39
308 9.13 5.66 2.82 9.24
313 9.02 5.72 2.82 9.28
318 9.01 5.81 2.82 9.42
323 8.74 5.82 2.82 9.30
C16TAB 303 11.41 6.13 5.08 37.00
308 11.95 6.35 3.98 33.54
313 11.71 6.40 2.86 29.59
318 12.43 6.66 1.72 26.36
323 12.23 6.72 0.56 22.54
PS40 303 2.85 2.64 0.82 11.40
308 2.75 2.59 3.55 19.94
313 3.07 2.92 6.32 29.52
318 3.03 2.93 9.14 37.98
323 3.29 3.20 12.01 47.08
PS60 303 3.74 3.32 3.69 23.16
308 3.97 3.53 3.95 24.29
313 4.04 3.63 4.21 25.07
318 3.95 3.63 4.48 25.49
323 4.28 3.90 4.75 26.78
PS80 303 3.51 3.16 8.90 39.80
308 3.60 3.28 10.37 44.31
313 4.01 3.61 11.86 49.43
318 4.36 3.89 13.37 54.29
323 4.45 4.01 14.91 58.60
[Water]/[surfactant] mole ratio, x = 15
Fig. 2 Interdependence of
-DGto on x and T for water/
(surfactant?n-butanol)/heptane
w/o microemulsion systems.
Surfactants: a C14TAB; b PS40.
Temp. (K): open squares, 303;
filled squares, 308; open circles,
313; filled triangles, 318 and
open triangles, 323
J Surfact Deterg (2010) 13:475–484 479
123
According to a previous report [27], DSto decreased with
increasing x both for ionic and nonionic surfactants for the
formation of microemulsions using alkanol as cosurfac-
tants. The comparative increase in DSto when the rise in
temperature was low. In this work, we have observed
almost unchanged DSto with temperature for C14TAB, its
lowering for C16TAB and increasing for C10TAB. The
observed trends remain unaccountable. The DHto values for
the cationics also follow the same trend as with DSto. For all
the nonionics DSto values clearly increase with temperature.
The DHto and DSt
o produce compensations with good cor-
relation for both classes of surfactants (Fig. 4), which was
also reported by others [42, 43]. The compensation tem-
peratures were 280 and 308 K obtained for the cationic and
the nonionic surfactant systems, respectively. The linear
compensation features comprising results at all x values
are extra thermodynamic correlation phenomenon.
Structural Parameters
The structural parameters computed from the experimental
data at x = 15 using Eqs. 14–23 are presented in Table 2
and Figs. 5, 6, 7. For both kinds of surfactants, Re increases
linearly with the surfactant chain length, also with x (as
expected). On the overall basis Re decreases with increas-
ing temperature (as shown in Fig. 5); the values range
between 1.66 and 3.30 nm in the temperature range of
303–323 K. They are comparable with the earlier reported
values for other systems [35, 37, 42]. The ratio of Re/Rw is
found to be independent of temperature, x, and surfactant
chain length (as revealed from Table 2). On average, the
Re/Rw ratio is found to be 1.55 for the ionic surfactants, and
2.60 for the nonionics. They were higher than in our earlier
report [27]. The types of surfactants used produce this
difference. For ionic surfactants a 50% reduction in droplet
size was noted while for nonionic surfactants it was only
17%.
The effective radii of the droplets decrease with the rise
in temperature and hence their number density, Nd,
increases. The Nd values decrease with increasing x for the
cationic systems but increase for the nonionic formulations
(Fig. 6). The latter results were not expected. This matter
requires further examination through experiments such as
dynamic light scattering measurements.
The population of surfactant (Ns) and cosurfactant (Na)
per droplet at the oil–water interface is found to decrease
with a temperature rise, whereas the Nd values increase.
Na–x–T profiles for C14TAB and PS40-containing micro-
emulsion are shown in Fig. 7 as representative plots.
Results are also summarized in Table 2. The results agree
with the literature reports [35, 37, 42]. For stabilization, in
the cationic surfactant-derived microemulsion systems,
Na [ Ns whereas Na � Ns in the nonionic-derived systems
(Table 2). These findings corroborate the existing literature
reports [35, 37, 42]. Like Re/Rw, temperature does not
affects the ratio Na/Ns either as shown in Table 2. For ionic
surfactants, however, Na/Ns decreases from 5.0 to 3.0 with
the variation of the surfactant from C10TAB to C16TAB.
Longer tail surfactants thus produce a better surface cov-
erage compared with the shorter analogues. However, for
the nonionic surfactants, such dependence was not found,
and Na/Ns remains almost constant at 35. Their bulkier
head group was considered to be responsible for producing
such an effect. For a clear elucidation further investigation
is warranted.
Conclusions
Dilution experiments for water/(surfactant?n-butanol)/
n-heptane w/o microemulsion at various temperatures and
[water]/[surfactant] mole ratios x, were performed. Two
sets of surfactants of the homologous series, alkyltrimethyl
ammonium bromides and polysorbates were used in the
Fig. 3 Variation of a DHto with surfactant carbon number on its
chain(Cn) and b DSto with the surfactant carbon number on its
chain(Cn) at different temperatures (K) at x = 15 for water/
(CnTAB(n = 10,14,16) ? n-butanol)/heptane w/o microemulsion
systems. Temp. (K): open squares, 303; filled squares, 308; opencircles, 313; filled circles, 318 and open triangles, 323
Fig. 4 DHto and DSt
o profiles for two different water/(surfactants?
n-butanol)/heptane w/o microemulsion systems. All x values (5, 10,
15, 20 and 25) have been used in the plots. a open circles, C10TAB;
open triangles, C14TAB; open squares, C16TAB; b inverted triangles,
PS40; diamonds, PS60 and filled circles, PS80. Compensation
temperatures for surfactants (K): Cationic, 292 and nonionic, 309
480 J Surfact Deterg (2010) 13:475–484
123
Table 2 Structural parameters of water/(CnTAB?n-butanol)/heptane w/o microemulsions under varied conditions
Surfactant Temp./K Re(Rw)/nm Re/Rw (Re-Rw)/nm 10-18 Nd (per mL) Na(Ns) (per droplet) Na/Ns
A. Cationic surfactants
C10TAB 303 2.92(1.86) 1.56 1.06 6.65 310(65) 4.8
308 2.58(1.63) 1.58 0.95 9.93 222(43) 5.2
313 2.23(1.41) 1.58 0.82 15.29 142(28) 5.1
318 1.90(1.19) 1.60 0.71 25.77 93(17) 5.5
323 1.60(1.01) 1.58 0.59 42.30 54(10) 5.4
C14TAB 303 3.29(2.18) 1.51 1.11 3.69 343(97) 3.5
308 2.87(1.91) 1.50 0.96 5.50 223(65) 3.4
313 2.41(1.59) 1.51 0.82 9.52 137(38) 3.6
318 1.99(1.32) 1.51 0.67 16.54 75(22) 3.4
323 1.64(1.09) 1.50 0.55 29.71 43(12) 3.6
C16TAB 303 3.45(2.29) 1.50 1.16 2.99 362(110) 3.3
308 3.00(2.00) 1.50 1.00 4.47 231(74) 3.1
313 2.50(1.66) 1.50 0.84 7.79 136(42) 3.2
318 2.05(1.37) 1.49 0.68 13.88 73(24) 3.0
323 1.68(1.12) 1.50 0.56 25.31 39(13) 3.0
B. Nonionic surfactants
PS40 303 2.65(1.01) 2.62 1.64 9.86 354(10) 35.4
308 2.53(0.96) 2.63 1.57 11.34 307(8) 38.4
313 2.42(0.91) 2.65 1.51 13.21 269(7) 38.4
318 2.32(0.89) 2.61 1.43 14.28 233(7) 33.3
323 2.22(0.84) 2.64 1.38 16.94 210(6) 35.0
PS60 303 2.69(1.03) 2.61 1.66 8.84 358(10) 35.8
308 2.54(0.97) 2.61 1.57 10.89 310(8) 38.8
313 2.44(0.95) 2.56 1.49 11.34 265(8) 33.1
318 2.34(0.93) 2.52 1.41 12.33 224(7) 32.0
323 2.23(0.88) 2.53 1.35 14.23 195(6) 32.5
PS80 303 2.67(1.02) 2.62 1.65 9.34 357(10) 35.7
308 2.55(0.97) 2.63 1.58 10.62 309(9) 34.3
313 2.43(0.92) 2.64 1.51 12.58 271(7) 38.7
318 2.33(0.89) 2.62 1.44 13.96 236(7) 33.7
323 2.23(0.86) 2.50 1.37 15.21 203(6) 33.8
At x = 15
Fig. 5 Dependence of Re on
x and T for water/(surfactants?
n-butanol)/heptane w/o
microemulsion systems at all
x values (5,10,15,20 and 25).
Surfactants: a C14TAB; b PS40.
Temp. (K): open squares, 303;
filled squares, 308; open circles,
313; filled triangles, 318 and
open triangles, 323
J Surfact Deterg (2010) 13:475–484 481
123
study. From the results, thermodynamic parameters on
the formation of w/o microemulsions were evaluated, and
the structural features of the nanodroplets of water formed
as dispersions in heptane were estimated. The results led to
the following conclusions:
1. Nonionic surfactants having bulkier head groups
require a higher amount of n-butanol to obtain a
microemulsion compared with the cationic surfactants.
2. The process is endothermic with a positive entropy
change. Enthalpy and entropy changes nicely com-
pensate for each other.
3. For the cationic surfactants the effective size of the
droplets increases with an increase in water pool size,
and decreases with a temperature rise. However, for
the nonionic surfactants, both water pool size and
temperature result in a lowering of the effective size of
the droplets.
4. The average aggregation number of surfactants per
droplet is lower for the nonionic surfactants because of
their larger head group dimension which is compen-
sated for by the accumulation of a larger number of
alkanol molecules. The reverse effect was noticed for
the cationic surfactants.
Acknowledgments Financial assistance from the University Grants
Commission, New Delhi, India, is thankfully acknowledged, and MD
acknowledges with appreciation the receipt of a fellowship from UGC
to execute the work. SPM acknowledges the support from the Indian
National Science Academy in the form of an Honorary Scientist
position and an Emeritus Professorship at the Jadavpur University.
References
1. Bourrel M, Schechter RS (1988) Microemulsions and related
systems. Dekker, New York
2. Solans C, Kunieda H (1997) Industrial applications of micro-
emulsions. Dekker, New York
3. Friberg SE, Bothorel P (eds) (1987) Microemulsions: structure
and dynamics. CRC, Boca Raton, Florida
4. Moulik SP, Paul BK (1998) Structure, Dynamics and Trans-
port Properties of Microemulsions. Adv Colloid Interface Sci
78:99–195
5. Paul BK, Mitra RK (2005) Water solubilization capacity of
mixed reverse micelles: effect of surfactant component, the nat-
ure of the oil, and electrolyte concentration. J Colloid Interface
Sci 288:261–279
6. Kumar P, Mittal KL (eds) (1999) Handbook of microemulsions
science and technology. Dekker, New York
7. Fanun M (2008) Microemulsions: properties and applications.
CRC Press, Boca Raton, Florida
8. Stubenrauch C (ed) (2009) Microemulsions—background, new
concepts, applications, perspectives. Blackwell, Oxford
Fig. 6 Dependence of Nd on xand T for water/(surfactants? n-
butanol)/heptane w/o
microemulsion systems at all xvalues (5,10,15,20 and 25).
Surfactants: a C14TAB and
b PS40. Temp. (K): opensquares, 303; filled squares,
308; open circles, 313; filledtriangles, 318 and opentriangles, 323
Fig. 7 Dependence of Na on xand T for water/(surfactants?
n-butanol)/heptane w/o
microemulsion systems at all
x values (5,10,15,20 and 25).
Surfactants: a C14TAB; b PS40.
Temp. (K): open squares, 303;
filled squares, 308; open circles,
313; filled triangles, 318 and
open triangles, 323
482 J Surfact Deterg (2010) 13:475–484
123
9. Schulman JH, Stoeckenius W, Prince LM (1959) Mechanism of
formation and structure of micro emulsions by electron micros-
copy. J Phys Chem 63:1677–1680
10. Chen SH, Rajagopalan R (eds) (1990) Micellar solutions and
microemulsions’: structure, dynamics and statistical thermody-
namics. Springer, New York
11. Hoar TP, Schulman JH (1943) Transparent water-in-oil disper-
sions: the oleopathic hydro-micelle. Nature 152:102–1033
12. Nagarajan R, Ruckenstein E (2000) Molecular theory of micro-
emulsions. Langmuir 16:6400–6415
13. Caponetti E, Martino DC, Floriano MA, Triolo R (1997) Local-
ization of N-alcohols and structural effects in aqueous solutions
of sodium dodecyl sulfate. Langmuir 13:3277–3283
14. Caponetti E, Lizzio A, Triolo R, Griffith WL, Johnson JS Jr
(1992) Alcohol partition in a water-in-oil microemulsion from
small-angle neutron scattering. Langmuir 8:1554–1562
15. Lagues M (1979) Electrical conductivity of microemulsions: a
case of stirred percolation. J Phys Lett 40:L331–L333
16. Lagues M, Ober R, Taupin C (1978) Study of structure and
electrical conductivity in microemulsions: evidence for percola-
tion mechanism and phase inversion. J Phys Lett 39:L487–L491
17. Petit C, Bommarius AS, Pileni MP, Hatton TA (1992) Charac-
terization of a four-component cationic reversed micellar system:
dodecyltrimethylammonium chloride/hexanol/n-heptane and
0.1 M potassium chloride solution. J Phys Chem 96:4653–4658
18. Bansal VK, Chinnaswamy K, Ramachandran C, Shah DO (1979)
Structural aspects of microemulsions using dielectric relaxation
and spin label techniques. J Colloid Interface Sci 72:524–537
19. Petit C, Bommarius AS, Pileni MP, Hatton TA (1992) Charac-
terization of a four-component cationic reversed micellar system:
dodecyltrimethylammonium chloride/hexanol/n-heptane and
0.1 M KCl solution. J Phys Chem 96:4653–4658
20. Hait SK, Moulik SP (2002) Interfacial composition and thermo-
dynamics of formation of water/isopropyl myristate water-in-oil
microemulsions stabilized by butan-1-ol and surfactants like cetyl
pyridinium chloride, cetyl trimethyl ammonium bromide, and
sodium dodecyl sulfate. Langmuir 18:6736–6744
21. Gerbacia W, Rosano HL (1973) Microemulsions: formation and
stabilization. J Colloid Interface Sci 44:242–248
22. Kumar S, Singh S, Singh HN (1986) Effect of chain length of
alkanes on water-in-oil microemulsions. J Surf Sci Technol 2:85–91
23. Birdi KS (1982) Microemulsions: effect of alkyl chain length of
alcohol and alkane. Colloid Polym Sci 26:628–631
24. Gu G, Wang W, Yan H (1998) Phase equilibrium and thermo-
dynamic properties in microemulsions. J Therm Anal Cal
51:115–123
25. Moulik SP, Aylward WM, Palepu R (2001) Phase behaviours and
conductivity study of water/CPC/alkan-1-ol (C4 and C5)/1-hexane
water/oil microemulsions with reference to their structure and
related thermodynamics. Can J Chem 79:1–13
26. Digout L, Bren K, Palepu R, Moulik SP (2001) Interfacial
composition, structural parameters and thermodynamic properties
of water-in-oil microemulsions. Colloid Polym Sci 279:655–663
27. De M, Bhattacharya SC, Panda AK, Moulik SP (2009) Interfacial
behavior, structure and thermodynamics of water in oil micro-
emulsion formation in relation to the variation of surfactant head
group and cosurfactant. J Dispersion Sci Technol 30:1262–1272
28. John AC, Rakshit AK (1995) Effects of mixed alkanols as co-
surfactants on single phase microemulsion properties. Colloids
Surf A 95:201–210
29. Antalek B, Williams AJ, Texter J, Feldman Y, Garti N (1997)
Microstructure analysis at the percolation threshold in reverse
microemulsions. Colloids Surf A 128:1–11
30. Lopez-Quintela MA, Tojo C, Blanco MC, Garcia Rio L, Leis JR
(2004) Microemulsion dynamics and reactions in microemul-
sions. Curr Opin Colloid Interface Sci 9:264–278
31. Lunkenheimer K, Miller R, Kretzschmar G, Lerche KH, Becht J
(1984) Investigations on the possibility of purifying surfactant
solutions by adsorption on solids. Colloid Polym Sci 262:662–
666 (and reference therein)
32. Gildnyi T, Stergiopoulos C, Wolfram E (1976) Equilibrium sur-
face tension of aqueous surfactant solutions. Colloid Polym Sci
254:1018–1023
33. Rosen MJ, Song LD (1996) Dynamic surface tension of aqueous
surfactant solutions 8. Effect of spacer on dynamic properties
of gemini surfactant solutions. J Colloid Interface Sci 179:261–
268
34. Bayrak Y (2004) Interfacial composition and formation of w/o
microemulsion with different amphiphiles and oils. Colloids Surf
A 247:99–103
35. Zheng O, Zhao JX, Yan H, Gao SK (2007) Dilution method study
on the interfacial composition and structural parameters of water/
C12-EOx-C12.2Br/n-hexanol/n-heptane microemulsion: effect of
the oxyethylene groups in the spacer. J Colloid Interface Sci
310:331–336
36. Lide DR (1999) Handbook of chemistry and physics, 80th edn.
CRC Press, Boca Raton, Florida
37. Zheng O, Zhao JX, Fu XM (2006) Interfacial composition and
structural parameters of water/C12-s-C12a2Br/n-hexanol/n-hep-
tane microemulsions studied by the dilution method. Langmuir
22:3528–3532
38. Singh HN, Swarup S, Singh RP, Saleem SM (1983) Structural
description of water-in-oil microemulsions using electrical
resistance. Ber Bunsen-Ges Phys Chem 87:1115–1120
39. Macia M, Seguer J, Infante MR, Selve C, Vinardell MP (1996)
Preliminary studies of the toxic effects of non-ionic surfactants
derived from lysine. Toxicology 106:1–9
40. Mitra D, Chakraborty I, Bhattacharya SC, Moulik SP, Roy S et al
(2006) Physicochemical studies on cetylammonium bromide and
its modified (mono-, di-, and trihydroxyethylated) head group
analogues. Their micellization characteristics in water and ther-
modynamic and structural aspects of water-in-oil microemulsions
formed with them along with n-hexanol and isooctane. J Phys
Chem B 110:11314–11326
41. Hait SK, Moulik SP (2001) Determination of critical micelle
concentration (CMC) of nonionic surfactants by donor–acceptor
interaction with iodine and correlation of CMC with hydrophile–
lipophile balance and other parameters of the surfactants. J Surf
Deterg 4:303–309
42. Paul BK, Nandy D (2007) Dilution method study on the
interfacial composition, thermodynamic properties and struc-
tural parameters of W/O microemulsions stabilized by 1-pent-
anol and surfactants in absence and presence of sodium
chloride. J Colloid Interface Sci 316:751–761 (and references
therein)
43. Wang F, Fang B, Zhang Z, Zhang S, Chen Y (2008) The effect of
alkanol chain on the interfacial composition and thermodynamic
properties of diesel oil microemulsion. Fuel 87:2517–2522 (and
references therein)
J Surfact Deterg (2010) 13:475–484 483
123
Author Biographies
Mrinmoy De received his M.Sc. degree in Chemistry from University
of Kalyani in 2003. He is doing his Ph.D. work under the supervision
of Dr Amiya Kr. Panda and Prof. Satya P. Moulik from Jadavpur
University. His area of research is in the field of surface science,
especially in microemulsions.
Subhash C. Bhattacharya is Professor of Chemistry and recently
Dean, Faculty of Science, Jadavpur University. He received his M.Sc.
and Ph.D. in Chemistry from the University of Burdwan and Jadavpur
University, respectively. His area of research includes surface science
and spectroscopic analysis.
Satya P. Moulik is an Emeritus Professor of Chemistry at Jadavpur
University, and Honorary Scientist of the Indian National Science
Academy at the Centre for Surface Science, Jadavpur University. He
is the Editorial Adviser of the Journal of Surface Science and
Technology, and the President of the Indian Society for Surface
Science and Technology. His research interests are surface and
biophysical chemistry with special reference to microheterogenous
systems, polymer (biopolymer)-surfactant interactions, synthesis, and
the characterization of nanoparticles and drug encapsulation and
delivery. He has published 300 original research papers.
Amiya K. Panda obtained his M.Sc. and Ph.D. degree in Chemistry
from Tripura University. He is an Associate Professor in Chemistry,
University of North Bengal. His area of research is in the fields of
polymer and surface science, nanomaterials and membrane mimetic
systems. He has so far published 40 papers in various journals.
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